The invention relates to a method and apparatus for preparing a plastic lens element having a gradient index of refraction. More specifically, the present invention is directed to a method and apparatus for manufacturing a lens wherein first and second monomer-containing compositions, each having a different index of refraction, are injected into a centrifugal mold at a controlled ratio to produce a lens element having a gradient index of refraction.
The concept of a lens without curved refracting surfaces dates to the turn of the century. R. W. Wood showed that a cylindrical shaped medium, such as a gelatin, with a refractive index changing radially outward from its central axis converges or diverges light although the faces of the medium are flat (R. W. Wood, Physical Optics, Macmillan, N.Y., 86-91, 1905). In addition, Wood proved mathematically that the ideal refractive index gradient would be parabolic.
While Wood's work demonstrated that a gradient index (GRIN) lens was possible, a commercially viable method of producing custom GRIN lenses at reasonable costs has not been developed to date.
Three basic factors must be considered when attempting to develop a commercially viable GRIN lens manufacturing process: producing a large enough index change; controlling the gradient-index shape; and reducing the time required to make a lens element. One established method of manufacturing a GRIN lens is to immerse a homogeneous alkali-silicate glass for predetermined time period in a molten salt bath containing a counter ion. In this binary diffusion process, ions from the molten salt migrate into the glass substrate and alkali ions from the glass exchange to enter the salt bath. The compositional change gives a change in refractive index as the exchange ions have a different polarization. A concentration gradient develops which in turn produces a gradient index profile. While the above-described process does produce a GRIN lens, the diffusion process, even at elevated temperatures, can take hours or days, depending on the size of the desired lens element. Thus, while the process may be commercially viable for very large volumes, it is not commercially practical for manufacturing customized or low volume GRIN lens elements.
Attempts to overcome the deficiencies of the above-described glass diffusion process have led to the processes for manufacturing plastic GRIN lens elements. U.S. Pat. No. 4,022,855 issued to Hamblen on May 10, 1977, and incorporated herein by reference, discloses a method for making a plastic optical element having a gradient index of refraction wherein two copolymerizable monomer compositions containing monomers having different indices of refraction are combined in a mold to form a polymerization mixture. The polymerization mixture has an index of refraction varying radially outward from its axis of rotation. Hamblen discloses two specific processes for forming the polymerization mixture. The first process involves injecting a first monomer composition into the mold at a high speed of rotation, thereby forcing the first monomer composition radially outward against the walls of the mold cavity before and during the insertion of a second monomer composition into the central mold cavity. The speed of rotation is reduced after the second monomer composition is injected into the mold and rotation is continued over a predetermined period of time to allow the desired degree of molecular diffusion of one monomer composition into the other.
The molecular diffusion process, however, can be almost as time consuming as the glass ion diffusion process previously described and, as is the case of any diffusion process, the reproducibility of results can be a problem. In addition, it is not possible to provide adjustment of the gradient index across the lens with a diffusion process, i.e. the gradient index is fixed by the diffusion process. For example, it would be desirable to produce a parabolic gradient index profile which is the most desirable shape for aberration correction. FIG. 1, however, shows a typical refractive index profile produced by an ion diffusion lens; following the complimentary error function shape of a concentration gradient. The low index tail region of the diffusion curve is not useful for aberration correction and must be masked out of the field by an aperture or placement outside the active light region. Various schemes have been tried to force the gradient shape towards parabolic curvature, but these schemes have been unsuccessful.
The second method disclosed in Hamblen addresses some of the deficiencies of the diffusion processes discussed above by feeding the two monomer compositions into the mold simultaneously, but at different proportional rates during the filling process. Thus, the refractive gradient occurs without waiting for the diffusion of the two monomers. The present invention provides an improvement of the second method disclosed in Hamblen which permits the gradient index to be adjusted to any desired shape, thereby overcoming all of the deficiencies of the diffusion processes described above.